It is often desirable to interface two printed circuit boards while maintaining them in a co-planar orientation. This requires a plurality of conductive jumpers to bridge the respective traces (or contact pads) of the two circuit boards, as well as a removable support assembly to lock the circuit boards together and to hold the jumpers in place against the appropriate traces or pads.
As circuit boards evolve toward higher densities, it is becoming increasingly difficult to complete a junction as described above. With current photolithographic techniques, the contact pads on a circuit board are precisely formed on the underlying board. Hence, the typical pitch between contact pads rarely exceeds 1 mm, and the tolerances in their positioning is very small. If reliable contact is to be made between the contact pads of two such boards, then the pads of both boards must be precisely aligned with the conductive jumpers which bridge the two boards. The need for precision is heightened even further when controlled impedance requirements are imposed on the junction.
Certain applications impose controlled impedance requirements ranging between 50 to 120 ohms for each point of electrical contact. Conventional impedance control is achieved by varying the following:
1. The shape of the contact pads or traces.
2. The distance between contact pads/traces.
3. The signal-to-ground structure.
4. The dielectric constant of the insulating board material.
In a co-planar PC board interface, all of the above-referenced factors are upset when the alignment is improper. As a result, the impedance constraints are more difficult to achieve.
Consequently, it remains a vexing problem to properly interface two co-planar high-density printed circuit boards, especially in a controlled impedance application.